Differential B and C fibre block by an amide- and an ester-linked local anaesthetic

The relative sensitivity of sympathetic preganglionic and postganglionic axons, B and C fibres, respectively, to two structurally dissimilar local anaesthetics was examined. The concentration of amethocaine (ester-linked tertiary amine) or prilocaine (amide-linked secondary amine) required to reduce the B fibre compound action potential by 50% was one-third of the concentration required to depress the C fibre potential to a similar extent. Both local anaesthetic agents also showed the action potential propagation through B fibres more than through C fibres. Similar results have been reported previously for lignocaine (amide-linked tertiary amine).